1. Field of Invention
This invention relates to a microtome object holder clamping structure for a ball-mounted specimen plate, and to a microtome object holder assembly comprising such structure.
2. Description of the Related Art
For histological analysis of samples and determination of their physiological and pathology characteristics, microtomes are utilized to produce extremely thin sections of a selected tissue specimen for microscopic examination.
For such sectioning, the microtome typically employs a specimen plate as a substrate element to which the tissue sample is affixed by an adhesive embedding medium. The specimen plate bearing such tissue sample then is clamped or otherwise fixedly positioned in the microtome apparatus, in proximity to a knife which is translatable into cutting contact with the mounted specimen, to yield thin cut sections of tissue for analysis.
In some instances, it is desirable to cryogenically fix the biological specimen by cooling it to low temperatures on the order of -100.degree. C. to about -190.degree. C. to produce a frozen "vitrified" sample for sectioning and analysis. Accordingly, cryostat microtomes have come into usage in which the microtome unit is mounted in a cooling enclosure. In such apparatus, the microtome is coupled to externally mounted controls to actuate and control the microtome. An illustrative cryostat microtome of such type is disclosed in U.S. Pat. No. 4,548,051 issued Oct. 22, 1985 to G. Moessner.
In recent years cryostat microtomes have become increasingly sophisticated, utilizing digital controls and monitoring devices affording a highly accurate, low-distortion sectioning operation. As an example, the Minotome.RTM. Digital Microtome-Cryostat, a cryostat microtome unit commercially available from International Equipment Company (Needham Heights, MA), is said to permit cutting of tissue sections with a thickness of from 2 to 42 microns, in precise 2 micron increments.
With such precision sectioning ability, it is critically important in the operation of the microtome that the mounted specimen be retained in a spatially fixed position, since any movement of the specimen relative to the precision controlled cutting blade will produce sections of uneven thickness. Such variability in turn introduces a distortion and possible error into the subsequent microscopic analysis of the tissue, and may lead to mis-characterization or mis-diagnosis of the tissue, or else require discarding of the sample and repetition of the sectioning effort.
In this respect, the bearing pressure exerted by the microtome knife on the sample is significant, particularly in the case of the aforementioned cryogenically frozen samples, which present a high cutting resistance relative to ambient temperature specimens. Thus, the shear, bearing, and torsional stresses variously exerted by the knife on the specimen may tend to cause the specimen plate to shift in its mounting structure, with the above-described adverse affects on uniform sectioning ability. It is therefore necessary that the specimen holder be mounted in a manner so that it resists any movement in the presence of such stresses.
At the same time, it is highly desirable, and in some instances critical, to provide the specimen plate with the ability to be selectively adjusted in position, so that the orientation of the microtome knife relative to the specimen mounted on the holder may be correspondingly varied.
These features--the requirement that the specimen plate be fixedly positioned and positionally constant during the sectioning operation, and the capability of the specimen holder to be selectively repositioned for subsequent sectioning operations--are frequently at odds with one another in the specimen plate assemblies which have been devised to date. In such systems, the adjustability of the specimen plate's position is accommodated by specimen plate mounting units which frequently are susceptible to slippage and movement as a result of the sectioning operation, so that it is difficult to maintain a desired uniformity of thickness of tissue specimens, particularly when the number of successive sections to be cut from the sample is large.
As an example of the foregoing difficulties, a widely employed microtome assembly is the International Minot Custom Microtome, commercially available from International Equipment Company (Needham Hights, MA), which features a specimen plate to which is threadably attached, on the side opposite the specimen mounting face of the plate, a ball and stem fitting. After attachment of the specimen plate to the ball and stem unit, the ball is introduced into a clamp device featuring an opening communicating with a generally spherical interior cavity. The ball of the specimen mounting assembly is inserted into this cavity. A set screw then is threadably advanced into the cavity so that its end surface bears compressively against the ball. This type of arrangement produces a "two point contact" of the ball, with one contact point being between the planar end surface of the set screw and the ball, and the other contact point being diametrally opposite the first contact point, where the ball bears against the interior surface of the receiving cavity. As a result of this two contact point arrangement, the specimen holder is susceptible to rotational movement about the contact points, as well as slippage caused by the forces exerted on the specimen during the cutting operation. Further, any low level vibration, movement, or adjustment of the microtome unit may cause the set screw to loosen and the ball to shift, along with the specimen plate and the specimen thereon.
Microtome specimen holder assemblies also have been utilized which feature a ball-mounted specimen plate held in fixed position by three opposedly arranged set screws. In such arrangement, a circumferential spacing of 120.degree. exists between the successive screws in the set screw array, so that a "three contact point" arrangement is provided when the respective set screws are threadably advanced into bearing contact with the ball element of the specimen holder. While this arrangement is effective to spatially fix the specimen, it has the attendant disadvantage that three set screws must be successively tightened to grip the ball. Manual repositioning of the specimen holder entails readjustment of these multiple screws, so that the positioning of this assembly is tedious and time-consuming in character.
Other set screw-type specimen plate or specimen holder mounting arrangements are shown in U.S. Pat. No. 2,155,523 issued Apr. 25, 1939 to E. Bausch et al and U.S. Pat. No. 3,091,144 issued May 28, 1963 to H. Fernandez-Moran Villalobos.
Alternative approaches which have been proposed inolude the magnetic mounting structure described in U.S. Pat. No. 3,190,164 issued June 22, 1965 to J. B. McCormick. In the system disclosed in that patent, the specimen is mounted on a magnetic chuck device, however such arrangement is subject to the same displacement and positional shifting disadvantages as the aforementioned mechanical mounting structures.
Accordingly, it would be a significant advance in the art to provide a specimen holder mounting structure which overcomes the above-described deficiencies.
It is therefore an object of the present invention to provide an improved microtome specimen holder mounting structure, which accommodates selective fixed positioning of a ball-mounted specimen plate in a manner which is highly resistant to movement during operation of the microtome, and which at the same time is readily selectively repositionable to different orientations.
It is another object of the present invention to provide a microtome specimen holder mounting structure of such type, which utilizes only a single adjustment element, and in which the ball-mounted specimen plate is readily adjustible, both rotationally and translationally, to a subsequently desired fixed position.
Other objects and advantages of the present invention will be more fully apparent from the ensuing disclosure and appended claims.